Swirling dust devils on Mars have given NASA scientists both a scientific treat and a very welcome power boost.

On 10 March, the rover Spirit captured images of two dust devils in one day. It is the first time any have been seen on Mars since first being identified in a single image from the Mars Pathfinder mission in 1997. One of the two appears on two different images from the rover's Navigation Camera, making it possible to track its direction and speed.

Furthermore, a separate dust devil has apparently swept the rover clean. The power output of the rover's solar panels had been reduced by almost half because of a year's worth of accumulated dust. But on 9 March, the output shot up to 93% of its initial level, giving it more power for future exploration.

Images looking down at the rover's deck show an almost pristine surface, with just a few small tails of dust, compared to the dingy surface seen just a day earlier.

The team is still figuring out exactly when the power boost occurred, and whether it was a single event or not. Science team member Geoffrey Landis told New Scientist that the cleaning of the solar panels may have taken place at night. But dust devils occur only in the midday sunlight, he notes, so it may have simply been a strong breeze that cleaned the rover.

Too much of a coincidence?

Matthew Golombek of NASA's Jet Propulsion Laboratory considers the breeze hypothesis implausible. Golombek, who was the chief scientist of the Pathfinder mission and is another member of the rover science team, told New Scientist : "We wait for months to see a dust devil and finally catch one and there's this big power boost within a day of it? That's too much of a coincidence."

The images are far from just a novelty. "We're getting some very interesting science" out of them, Landis said. For example, it is the first time they have had a way to measure wind speeds directly, since there is no anemometer onboard. The two images of the same dust devil show that it was moving at an average speed of 3 metres per second.

If more pictures are found that show dust devils, it might be possible to gather meaningful statistics on their frequency and duration. "That would be very exciting," says Landis.

But already, as far as atmospheric science is concerned, this detection "is certainly the highlight of the mission so far", he says.